Digital-subscriber line (DSL) technology provides users with high-speed data connections over an ordinary telephone line. Modern DSL systems use various methods to modulate data and communicate over this telephone line. One such method of modulation, multi-carrier modulation, divides DSL's frequency band into several channels. Discrete Multi-Tone (DMT) modulation is one popular type of multi-carrier modulation.
Like many other modern technologies, the challenges associated with developing DSL services have been significant. Because DSL is often used over ordinary telephone lines (i.e., pairs of twisted copper wire), limiting the effect of noise has been a particular challenge. Crosstalk is one type of noise wherein the electrical signals on adjacent wires interfere with one another. Crosstalk depends on the number of DSL systems turned ON in a bundle of wires (e.g., cable binder). For example, during the night, the number of DSL systems turned ON may relatively low and, accordingly, crosstalk may be relatively low. In contrast, during late afternoon when businesses are operating and when students have returned home from school, the number of DSL systems turned ON may be relatively high and crosstalk may be relatively high. In addition to crosstalk, other types of non-stationary noise may be occasionally present in DSL systems, including RFI, impulse noise, and numerous others.
Because such noise may cause errors in transmitted data, several methods are known that attempt to reduce the effect of such noise in modern DSL systems, particularly with respect to how bits are loaded onto the several channels in DMT. One traditional method is for a modem to transmit data with extra signal-to-noise ratio (SNR) margin. In this method, one modem measures the SNR of a received signal during initialization, and then transmits data at a higher power (extra SNR margin) to ensure that the data is communicated error free. Alternatively, the modem may use the same transmit power, but increase the SNR margin by loading fewer bits of data onto each subcarrier. In one common embodiment, the transmitting modem will add an extra 6 dB of SNR margin. Because this extra SNR margin is usually based on a measurement that is made during initialization, it may be insufficient to account for the actual noise encountered during data communication due to variations in the noise environment.
An unpleasant situation can occur when a subscriber's modem goes through initialization with no other systems operating in the cable binder. In such a situation, the subscriber's modem determines that there is a relatively low level of noise on the line and transmits data at a relatively low power. As other modems in the binder are turned ON, the noise on the line may increase (e.g., due to crosstalk). Because the subscriber's modem usually determines the bit loading by estimating the SNR of the received signal only at initialization, the modem cannot account for the increase of noise coming after the initialization from the additional modems. Ultimately, the modem's SNR will fall below acceptable levels and excess errors will occur, causing communication failure. In less dramatic cases, the modem will loose its SNR margin and an on-line reconfiguration process can reduce the bit rate accordingly.
In some implementations modems can adjust their SNR margin-using dynamic power adjustment or dynamic bit loading algorithms, such as on-line bit rate adaptation. However, because these methods require coordination of the adjustments between the modems (which may be impossible due to high error rate in the channel), these methods are often insufficient to avoid communication failure.
Although there are many specific strategies developed for DMT bit loading, these strategies are insufficient to account for noise variations coming from a variety of sources. As a result, an improved method to account for noise from a variety of sources is needed.